NEW YORK (AP) — Facebook's stock price is falling now that employees are able start selling restricted stock in the company.

A lock-up period that had prevented such sales expired on Monday. U.S. stock markets opened on Wednesday for the first time since Superstorm Sandy hit the East Coast, so that's when employees could start selling.

Lock-ups are common after initial public stock offerings and are designed to prevent a stock from experiencing the kind of volatility that might occur if too many shareholders decide to sell at once.

Facebook Inc.'s stock is down 94 cents, or 4.3 percent, to $21 in morning trading.